Electrode for plasma arc torch and method of fabrication

ABSTRACT

An electrode for plasma arc torches has a tip body having a tip cavity with a cavity sidewall. An emissive insert having an insert sidewall resides in the tip cavity. A noble metal foil is interposed between the insert sidewall and the cavity sidewall. To fabricate the electrode, a tip body blank is provided, and a tip cavity having a cavity sidewall is formed therein. An emissive insert having an insert sidewall and being loosely insertable into the tip cavity is provided. The insert sidewall is provided with a textured surface with protuberances. A noble metal foil is selected, having an insert-contacting surface and a cavity-contacting surface. The emissive insert and the noble metal foil are placed into the tip cavity of the tip body blank, arranged such that the insert-contacting surface of the foil faces the insert sidewall of the emissive insert, while the cavity-contacting surface of the foil faces the cavity sidewall. After the emissive insert and the foil have been placed in the tip cavity, the tip body blank is placed into a die and radially deformed to compress the tip body blank and force the metal foil to conform to the textured surface of the insert sidewall. The deformed tip body blank is then machined to a final electrode configuration.

FIELD OF INVENTION

The present invention relates to a plasma arc electrode for use inplasma arc torches and, in particular, to an electrode which can bereadily fabricated and provides a long service life, as well as a methodfor making the same.

BACKGROUND OF THE INVENTION

Plasma arc torches are commonly used when fabricating metal structures.They are frequently employed in operations such as cutting, welding,treating surfaces, and annealing. These torches include an electrodewhich supports an arc which is struck between the electrode and theworkpiece. The arc is sustained therebetween when the torch is in thearc transfer mode of operation. The electrodes currently used in manyplasma arc torches have an electrode tip body fabricated from a highconductivity material such as copper or a copper alloy. An emissiveinsert is placed in the tip of the electrode. These emissive inserts arefabricated from materials with a low work function, and are frequentlymade from hafnium, zirconium, or tungsten. Further discussion of theseelectrodes is found in U.S. Pat. No. 5,023,425, which also notes thatsuch electrodes have short lives, since the arc may become supportedfrom the tip body of the electrode, rather than from the emissiveinsert, causing deterioration of the tip body. To extend the life ofsuch electrodes, the '425 patent teaches providing a sleeve having aradial thickness of at least about 0.01 inches positioned between theemissive insert and the copper or copper alloy tip body of theelectrode. This sleeve is fabricated from a metallic material having awork function which is greater than that of the material of the emissiveinsert, and maintains the arc being supported by the emissive insert.The '425 patent suggests the use of a variety of noble metals for thesleeve, and provides examples of electrodes employing silver sleeves.

The '425 patent teaches a method for fabricating such an electrode whichincludes the following steps. A first blank of copper or copper alloy isprovided, having a front face. A cavity is bored in the front face. Asecond blank of silver is formed and is metallurgically bonded into thecavity of the first blank by use of a brazing material. An opening isthen drilled into the second blank, and an emissive insert is forcefitted into the drilled opening. The assembly is then machined toprovide a smooth front face for the assembly.

U.S. Pat. No. 5,200,594 teaches an alternative solution to the problemof short electrode life by employing a plated emissive insert. Theemissive insert is first plated with nickel and thereafter plated with anoble metal.

While the solutions of both of the above mentioned patents provide anelectrode with a longer life, they complicate the process of fabricatingthe electrode. Furthermore, the electrodes of the '425 patent requiresubstantial quantities of silver to fabricate, much of which is machinedaway during the manufacturing of the electrode. The method of the '594patent reduces the quantity of the high work function metal which isrequired; however, it requires a double plating process where anintermediate layer of nickel is deposited onto the insert before platingwith a noble metal. Furthermore, the '594 patent teaches that the use ofa nickel plate is required to assure adhesion of plated layers duringthe subsequent processing. The adhesion is reported to be important tomaintain a good thermal path for dissipating the heat generated in use,thereby extending the life of the electrode.

Thus, there is a need for an electrode which can be readily fabricatedwith little waste and which will provide a long service life.

SUMMARY OF THE INVENTION

The present invention relates to an electrode for use in plasma arctorches and a method for making the same. These electrodes have a tipbody which is typically fabricated from copper or a copper alloy. Thetip body is symmetrically disposed about a tip central axis andterminates in a torch engaging end and a free terminating surface. Thefree terminating surface is substantially normal to the tip centralaxis. The tip body is configured with a tip cavity, having a cavitysidewall which extends inward from the free terminating surface and issymmetrically disposed about the tip central axis. An emissive inserthaving an insert sidewall which is symmetrically disposed about aninsert axis resides in the tip cavity and is positioned such that theinsert axis and the tip central axis are substantially coincident.

The method of fabricating an electrode of the present invention isinitiated by providing a tip body blank, which is typically eithercopper or a copper alloy. The tip body blank has a blank central axisand a free terminating surface which is substantially normal to theblank central axis.

A tip cavity is formed in the free terminating surface of the tip bodyblank. The tip cavity is configured such that it has a cavity sidewallwhich is symmetrically disposed about the blank central axis of the tipbody blank.

An emissive insert is provided, having a textured insert sidewallsymmetrically disposed about an insert axis. The emissive insert isconfigured to be loosely insertable into the tip cavity of the tip bodyblank, such that the insert axis is substantially coincident with thecentral axis of the tip body blank when the emissive insert is soinserted. The insert sidewall has a textured surface with protuberancesincorporated therein to increase the effective surface area of theinsert sidewall. Preferably, the protuberances are sufficient in sizeand number to increase the effective surface area by at least about 30%.Such a textured surface can typically be provided by grooves or byetching the surface of the insert sidewall.

A noble metal foil is selected, the foil having an insert-contactingsurface and a cavity-contacting surface which are spaced apart by athickness T of less than about 0.01 inches. While a variety of noblemetal foils can be employed, including silver, gold, platinum, andrhodium, it is preferred that the foil be silver and it is furtherpreferred that the silver foil have a purity of at least 99.5% byweight, and more preferably 99.7% by weight.

The emissive insert and the noble metal foil are placed into the tipcavity of the tip body blank, arranged such that the insert-contactingsurface of the foil faces the insert sidewall of the emissive insert,while the cavity-contacting surface of the foil faces the cavitysidewall. The foil is of sufficient size to substantially surround theinsert sidewall, providing a foil-wrapped insert.

When the emissive insert has a diameter of greater than about 0.06inches, the insert-contacting surface of the noble metal foil ispreferably wrapped around the insert sidewall to provide a foil-wrappedinsert prior to insertion into the tip cavity. The tip cavity, thethickness T of the foil, and the emissive insert are sized such that thefoil-wrapped insert can be readily inserted into the tip cavity. Sincethere is an elastic component to the deformation of the foil when it isbent around the emissive insert, it is frequently necessary to maintainthe foil in position around the insert sidewall until the foil-wrappedinsert is placed into the tip cavity.

Alternatively, when the emissive insert has a diameter of less thanabout 0.06 inches, the foil is preferably rolled, with thecavity-contacting surface facing outwards, and inserted into the tipcavity prior to inserting the emissive insert. Again, the elasticcomponent of the deformation of the foil provides a spring like action.When the rolled foil is inserted into the tip cavity and released, theelasticity of the foil forces the cavity-contacting surface of thecoiled foil into contact with the cavity sidewall. Again, the tipcavity, the thickness T, and the emissive insert are sized such thatthere is sufficient clearance to allow the insertion of the emissiveinsert into the foil-lined tip cavity to provide a foil-wrapped insert.

In either case, after the emissive insert and the foil have been placedin the tip cavity, the tip body blank is placed into a die and deformednormal to the tip central axis, providing a quasi-isotropic compressionof the tip body blank.

When the tip body blank is so compressed, the cavity wall of the tipbody blank exerts a force on the metal foil so that it conforms to thetextured surface of the insert sidewall, thereby insuring intimate andextended contact between the insert-contacting surface of the metal foiland the insert sidewall. Since the foil is thin, the thickness T beingless than about 0.01 inches and preferably less than about 0.06 inches,the deformation of the foil results principally from bending, and thecavity-contacting surface remains substantially parallel to theinsert-contacting surface. This results in a textured cavity-contactingsurface of the foil, which assures that there is intimate and extendedcontact between the cavity sidewall and the cavity-contacting surface.By maintaining the thickness T of the foil at a value greater than about0.001 inches, and preferably greater than about 0.002 inches, the foilhas sufficient strength to withstand lateral stresses induced by thedeformation of the foil so as to maintain the integrity of the foil toavoid rupturing of the foil and loss of its continuity about theemissive insert.

The deformed tip body blank is then machined to a final configuration.The machining typically includes a turning operation, to remove anylateral seams introduced by the dies used to produce the deformation,and facing of the free terminating surface to assure that thefoil-wrapped insert forms an integral part of the free terminatingsurface.

The electrode of the present invention, which can be fabricated by theabove method, includes the standard elements of a tip body and anemissive insert. The tip body is typically copper or a copper alloy, andis symmetrically disposed about a tip central axis. The tip bodyterminates in a torch engaging end and free terminating surface, thefree terminating surface being substantially normal to the tip centralaxis. The tip body has a tip cavity, having a cavity sidewall whichextends inwards from the free terminating surface and is symmetricallydisposed about the tip central axis.

The emissive insert has an insert sidewall which is symmetricallydisposed about an insert axis. The insert sidewall is a texturedsurface, having protuberances incorporated therein. The emissive insertresides in the tip cavity and is positioned such that the insert axisand the tip central axis are substantially coincident.

A noble metal foil is located between the cavity sidewall and the insertsidewall. The noble metal foil has a thickness T and terminates in aninsert-contacting surface and a cavity-contacting surface. The foil ismaintained in engaging contact with the cavity sidewall and the insertsidewall such that the insert-contacting surface is in direct andintimate contact with the insert sidewall of the emissive insert, andthe cavity-contacting surface is in direct and intimate contact with thecavity sidewall of the tip body.

It is preferred that the thickness T of the foil be less than about 0.01inches, and more preferably be further limited to less than about 0.006inches, to assure direct and intimate contact between thecavity-contacting surface of the noble metal foil and the cavitysidewall of the tip body.

It is also preferred that the thickness T of the foil be at least about0.001 inches, and more preferably at least about 0.002 inches, toenhance the effectiveness of the metal foil in extending the life of theresulting electrode by assuring continuity of the foil.

While the foil can be selected from a variety of the noble metals andalloys such as silver, gold, platinum, and rhodium, the preferred metalis silver which has a high resistance to oxidation in combination with ahigh thermal conductivity. It is further preferred that the silver havea purity level of at least 99.93% and more preferably 99.95% by weight.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 through 5 illustrate a sequence of steps for a method forfabricating an electrode of the present invention. FIG. 1 is an explodedview showing the components which form the electrode. FIG. 2 illustratesthe components assembled into an electrode assembly. FIG. 3 illustratesthe electrode assembly being pressed in a die to provide engagingcontact of the noble metal foil with the cavity sidewall and the insertsidewall. FIG. 4 illustrates the electrode assembly after pressing. FIG.5 illustrates the finished electrode assembly after machining.

FIGS. 6 and 7 illustrate steps for one method of inserting an emissiveinsert and a foil into a tip cavity of an electrode assembly.

FIGS. 8 and 9 illustrate alternative steps for assembling the emissiveinsert and the foil into the tip cavity of the electrode assembly.

FIG. 10 is a longitudinal cross section view of an electrode of thepresent invention. The electrode has a tip body having a central axisand a free terminating surface substantially normal to the central axis.A tip cavity having a cavity sidewall resides in the tip body andextends to the free terminating surface. An emissive insert has aninsert sidewall which is surrounded by a noble metal foil. The emissiveinsert and the foil are positioned in the tip cavity such that the foilis engaged with both the cavity sidewall and the insert sidewall.

FIG. 11 is a view from the plane 11—11 of FIG. 10, illustrating theradial spatial relationship of the tip body, the foil, and the emissiveinsert.

FIG. 12 a is a detail view of the region 12 of FIG. 11, illustratingdetails of one embodiment of the electrode of the present invention. Inthis embodiment, protuberances in the insert sidewall are provided byforming longitudinal grooves in the surface. The foil conforms to thesegrooves, providing interlocking engaging contact between the insertsidewall and the foil, and between the foil and the cavity sidewall.

FIG. 12 b is a detail view showing the same region as FIG. 12 a, butwhere the surface of the insert sidewall has been etched to provide amottled surface to which the foil conforms.

BEST MODE OF CARRYING THE INVENTION INTO PRACTICE

FIGS. 1 through 5 illustrate steps in a method for fabrication of anelectrode of the present invention. FIG. 1 is an assembly drawing of anelectrode assembly blank 100. An electrode body blank 102 is provided.The electrode body blank 102 has a central axis 104 and terminates in atorch engaging end 106 and in a free terminating surface 108, which issubstantially normal to the central axis 104. A tip cavity 110 isdrilled or counterbored into the free terminating surface 108, providingthe tip cavity 110 with a cavity sidewall 112 that is symmetricallydisposed about the central axis 104.

An emissive insert 116 having an insert sidewall 118 symmetricallydisposed about an insert axis 120 is provided. The emissive insert 116is formed from a metal which has a high work function, and preferably isformed of hafnium, tungsten, or zirconium. The emissive insert 116 isconfigured so as to be loosely insertable into the tip cavity 110 whenthe insert axis 120 is substantially aligned with central axis 104.

The insert sidewall 118 is textured, providing protuberances (not shown)therein. The protuberances are discussed in greater detail below andillustrated in FIGS. 12 a and 12 b. The protuberances preferablyincrease the effective surface area of the insert sidewall 118 by atleast about 30%. The textured surface may be provided by forming theemissive insert 116 from extruded stock which has been extruded througha die which is configured to leave a series of longitudinal grooves onthe insert sidewall 118. Alternatively, the surface of the insertsidewall 118 can be etched to form a mottled surface havingprotuberances.

A foil 122 is selected, which has an insert-contacting surface 124 and acavity-contacting surface 126, separated by a thickness T. The foil 122may be any of a variety of noble metals, including metals and alloys ofsilver, gold, platinum, and rhodium. If an alloy is selected, it isfurther preferred that it be a single phase alloy, and still furtherpreferred that the alloy be a low alloy material (e.g., the base metalat least 99% of the total weight). It is further preferred that the foil122 be silver to provide a high degree of resistance to oxidization incombination with a high thermal conductivity, and more preferably is atleast 99.93% silver by weight.

The thickness T of the foil 122 is selected to be not less than about0.001 and not greater than about 0.01 inches to assure proper plasticbehavior of the foil 122, as is discussed in greater detail below.

FIG. 2 illustrates the electrode body blank 102 into which the emissiveinsert 116 and the foil 122 have been inserted to form the electrodeassembly blank 100. The emissive insert 116 and the foil 122 arearranged such that the foil 122 surrounds the emissive insert 116, withthe insert-contacting surface 124 of the foil 122 facing the insertsidewall 118. The cavity-contacting surface 126 of the foil 122 thusfaces the cavity sidewall 112 of the electrode body blank 102.

FIG. 3 illustrates the electrode assembly blank 100, having the emissiveinsert 116 surrounded by the foil 122 positioned in the tip cavity 110,loaded into a multi-part die 132 having jaws 134 which radially compressthe electrode body blank 102 about the emissive insert 116 and the foil122. The multi-part die 132 typically has three to eight jaws. Theelectrode body blank 102 is copper or a copper-based alloy which hassufficient malleability to be plastically deformed by the die 132 tomake engaging contact with the foil 122.

The foil 122 is selected to have a combination of thickness andmalleability such that, when the electrode body blank 102 is radiallycompressed, the insert-contacting surface 124 of the foil 122 makesengaging contact with the insert sidewall 118, and the cavity-contactingsurface 126 makes engaging contact with the cavity sidewall 112. Placingan upper limit on the thickness T at about 0.01 inches assures that thefoil 122 remains sufficiently thin for the insert-contacting surface 124and the cavity-contacting surface 126 of the foil 122 to remainsubstantially parallel during deformation of the foil 122 between theinsert sidewall 118 and the cavity sidewall 112. Placing a lower limiton the thickness T of about 0.001 inches assures that the deformation ofthe foil 122 does not result in any substantial discontinuity of thefoil 122 between the insert sidewall 118 and the cavity sidewall 112,thereby assuring effective performance of the foil 122 in preventingtransfer of an arc from the emissive insert 116 to the electrode bodyblank 102.

It is preferred that the foil 122 be selected to have the thickness T nogreater than about 0.006 inches to enhance the parallel relationship ofthe insert-contacting surface 124 and the cavity-contacting surface 126.It is further preferred that the foil 122 be selected to have thethickness T no less than about 0.002 inches to assure continuity of thefoil 122 between the insert sidewall 118 and the cavity sidewall 112.

Preferably, the magnitude of radial deformation of the electrode bodyblank 102 should be greater than the difference between the diameter ofthe tip cavity 110 and the combined diameter of the emissive insert 116and the foil 122. For example, when the emissive insert 116 has adiameter of 0.060 inches and the foil 122 has a thickness of 0.002inches, providing an effective combined diameter of 0.064 inches, thecavity diameter is slightly larger to allow inserting the emissiveinsert 116 and the foil 122 thereinto. For this example, the tip cavity110 would typically have a diameter of about 0.074 inches, providing aneffective gap of 0.005 inches. For such dimensions, compression of theelectrode body blank 102 sufficient to cause a radially inwarddeformation of about 0.020 inches has been found to be effective inproviding engaging contact between the insert-contacting surface 124 andthe insert sidewall 118, and between the cavity-contacting surface 126and the cavity sidewall 112.

FIG. 4 illustrates a pressed electrode assembly blank 140 which haslateral ridges 142 resulting from the pressing in the die 132. Thepressed electrode assembly blank 140 is then machined to eliminate thelateral ridges 142 and to contour the free terminating surface 108,providing a finished electrode 150 as illustrated in FIG. 5.

FIGS. 6 and 7 illustrate one set of steps used for placing the emissiveinsert 116 and the foil 122 into the electrode assembly blank 100 whenthe emissive insert 116 is relatively large, having a diameter ofgreater than about 0.06 inches. FIG. 6 illustrates the insert-contactingsurface 124 of the foil 122 being wrapped around the insert sidewall 118of the emissive insert 116 to form a foil-wrapped insert 160.

As indicated in FIG. 7, the wrapped insert 160 is then placed into thetip cavity 110 of the electrode body blank 102. This arranges theemissive insert 116 and the foil 122 such that the insert-contactingsurface 124 of the foil 122 faces the insert sidewall 118, and thecavity-contacting surface 126 of the foil 122 faces the cavity sidewall112.

FIGS. 8 and 9 illustrate alternative steps for placing the emissiveinsert 116 and the foil 122 into the electrode assembly blank 100, whenthe emissive insert 116 is relatively small and has a diameter of lessthan about 0.06 inches. As shown in FIG. 6, the foil 122 is rolled up,with the cavity-contacting surface 126 facing outwards. The foil 122 isrolled to a foil diameter d which is less than a cavity diameter D ofthe tip cavity 110. The rolled foil 122 is then inserted into the tipcavity 110. Typically, the foil 122 has a degree of elasticity, and wheninserted expands until the cavity contacting surface 126 contacts thecavity sidewall 112.

As indicated in FIG. 9, the emissive insert 116 is then inserted intothe tip cavity 110 such that the insert-contacting surface 124 of thefoil 122 surrounds the insert sidewall 118.

FIG. 10 is a longitudinal cross section of a plasma arc electrode 200 ofthe present invention, which is fabricated by the methods illustrated inFIGS. 1-9. The plasma arc electrode 200 has an electrode tip body 202having a central axis 204 and terminating in a torch engaging end 206and a free terminating surface 208. The free terminating surface 208 issubstantially normal to the central axis 204. The torch engaging end 206frequently has a cooling chamber or passage 210 therein, through which acooling fluid circulates to cool the electrode 200 during use. Theelectrode tip body 202 has a tip cavity 212 having a cavity sidewall214. The cavity sidewall 214 is symmetrically disposed about the centralaxis 204. The cavity sidewall 214 extends to and terminates in the freeterminating surface 208 of the electrode tip body 202.

An emissive insert 220 is provided, which is fabricated from a metalwhich has a high work function, preferably hafnium, tungsten, orzirconium. The emissive insert 220 has an insert sidewall 222 which issymmetrically disposed about an insert axis 224. The insert axis 224 iscoincident with the central axis 204.

The insert sidewall 222 is textured, having protuberances 226incorporated therein. The protuberances 226 may be induced whileextruding the emissive insert 220 by passing the emissive insert 220through a die configured to leave a series of longitudinal grooves 228on the insert sidewall 222, as shown in FIG. 12 a. Frequently, suchtexturing is provided when extruding stock from which the emissiveinsert 220 is cut. Alternatively, the insert sidewall 222 can be etchedto provide a mottled surface having the protuberances 226, as shown inFIG. 12 b.

Interposed between the cavity sidewall 214 and the insert sidewall 222is a noble metal foil 230. The noble metal foil 230 has aninsert-contacting surface 232 and a cavity-contacting surface 234, whichare separated by a thickness T. The insert-contacting surface 232 of thenoble metal foil 230 is in engaging contact with the insert sidewall 222of the emissive insert 220, while the cavity-contacting surface 234 ofthe noble metal foil 230 is in engaging contact with the cavity sidewall214 of the electrode tip body 202. The character of the engaging contactis determined, in part, by the properties of the insert sidewall 222,the thickness T of the foil 230, and the character of the material ofthe foil 230.

It is preferred that the noble metal of the foil 230 be silver, which ishighly resistant to oxidation and is highly thermally conductive. It isfurther preferred that the noble metal be at least 99.93% silver byweight.

The thickness T of the foil 230 should be between about 0.001 and 0.01inches to assure proper plastic behavior of the foil 230. The upperlimit of thickness assures that the foil 230 remains sufficiently thinto assure that the insert-contacting surface 232 and thecavity-contacting surface 234 of the foil 230 remain substantiallyparallel after deformation of the foil 230 between the insert sidewall222 and the cavity sidewall 214. The lower limit of thickness assuredthat the deformation of the foil 230 does not result in discontinuity ofthe foil 230 between the insert sidewall 222 and the cavity sidewall214.

It is preferred that the thickness T of the foil 230 be maintained lessthan about 0.006 inches to enhance the parallel relationship of theinsert-contacting surface 232 and the cavity-contacting surface 234. Itis further preferred that the thickness T of the foil 230 be maintainedgreater than about 0.002 inches to more positively assure continuity ofthe foil 230 for more effective performance in preventing transfer of anarc from the emissive insert 220 to the electrode tip body 202.

While the novel features of the present invention have been described interms of particular embodiments and preferred applications, it should beappreciated by one skilled in the art that substitution of materials andmodification of details obviously can be made without departing from thespirit of the invention.

What I claim is:
 1. An electrode for use in plasma arc torches, theelectrode comprising: a tip body fabricated from copper or a copperalloy, said tip body having a central axis and a free terminatingsurface substantially normal to said central axis; a tip cavity having acavity sidewall, said tip cavity residing in said tip body and extendingto said free terminating surface; an emissive insert having an insertsidewall which is textured to provide a textured surface, said insertsidewall being substantially symmetrical about said central axis of saidtip body and said emissive insert forming part of said free terminatingsurface when said emissive insert is positioned in said tip cavity; afoil of a noble metal interposed between said cavity sidewall and saidinsert sidewall, said foil having an insert-contacting surface, which isin interlocking, engaging contact with said textured surface of saidinsert sidewall, and a cavity-contacting surface, which is substantiallyparallel to said insert-contacting surface so as to be in interlocking,engaging contact with said cavity sidewall, said insert-contactingsurface and said cavity-contacting surface being separated by athickness T of less than about 0.01 inches.
 2. The electrode of claim 1wherein said thickness T is between about 0.002 and 0.006 inches.
 3. Theelectrode of claim 2 wherein said foil is silver.
 4. The electrode ofclaim 3 wherein said silver has a purity of at least about 99.93%. 5.The electrode of claim 4 wherein said textured surface is provided byscoring the surface to form a series of grooves.
 6. The electrode ofclaim 4 wherein said textured surface is formed by etching to form amottled surface.
 7. The electrode of claim 1 wherein said texturedsurface increases the surface area of said insert sidewall by at leastabout 30%.
 8. A method for fabricating an electrode having an emissiveinsert for use in plasma arc torches, the method comprising the stepsof: providing a copper tip blank, the tip blank having a central axisand a free terminating surface substantially normal to the central axis;boring a tip cavity in the free terminating surface of the tip blank,the tip cavity having a cavity sidewall symmetrically disposed about thecentral axis of the tip blank; providing the emissive insert, theemissive insert having an insert sidewall symmetrically disposed aboutan insert axis, the emissive insert being configured so as to be looselyinsertable into the tip cavity and, when so inserted, positioned suchthat the insert axis is substantially coincident with the central axisof the tip blank; texturing the insert sidewall; providing a noble metalfoil having an insert-contacting surface and a cavity-contacting surfaceseparated by a thickness which allows the foil to be interposed betweenthe insert sidewall and the cavity sidewall; positioning the emissiveinsert and the foil into the tip cavity, the foil being interposedbetween the insert sidewall and the cavity sidewall with theinsert-contacting surface of the foil facing the insert sidewall of theemissive insert, and the cavity-contacting surface of the foil facingthe cavity sidewall of the tip blank; deforming the tip blanksufficiently to assure interlocking engaging contact of theinsert-contacting surface of the foil with the textured insert sidewallof the emissive insert, and of the cavity-contacting surface of the foilwith the cavity sidewall of the tip blank, whereby the insert-contactingsurface of the foil becomes mechanically interlocked with the texturedinsert sidewall of the emissive insert, and the cavity-contactingsurface of the foil becomes mechanically interlocked with the cavitysidewall; and machining the tip blank to form the electrode tip.
 9. Themethod of claim 8 wherein the emissive insert has a diameter of greaterthan about 0.06 inches, further wherein said step of positioning theemissive insert and the foil into the tip cavity further comprises:wrapping the insert-engaging surface of the foil around the insertsidewall to provide a foil-wrapped insert; and positioning thefoil-wrapped insert into the tip cavity.
 10. The method of claim 9 wherethe foil is chosen to be silver having a purity of at least 99.3% silverby weight, and further wherein said step of texturing the insertsidewall further comprises: passing the emissive insert through a dieconfigured to provide longitudinal grooves.
 11. The method of claim 9where the foil is chosen to be silver having a purity of at least 99.3%silver by weight, and further wherein said step of texturing the insertsidewall further comprises: etching the insert sidewall to provide amottled surface of the insert sidewall.
 12. The method of claim 8wherein the emissive insert has a diameter of less than about 0.06inches, further wherein said step of positioning the emissive insert andthe foil into the tip cavity further comprises: rolling the foil withthe cavity-contacting surface facing outwards to a diameter less thanthat of the tip cavity; inserting the rolled foil into the tip cavityand allowing the foil to expand such that the cavity-engaging surface ofthe foil resides against the cavity sidewall; and positioning theemissive insert into the tip cavity such that the foil is interposedbetween the insert sidewall and the cavity sidewall.
 13. The method ofclaim 12 where the foil is chosen to be silver having a purity of atleast 99.3% silver by weight, and further wherein said step of texturingthe insert sidewall further comprises: passing the emissive insertthrough a die configured to provide longitudinal grooves.
 14. The methodof claim 12 where the foil is chosen to be silver having a purity of atleast 99.3% silver by weight, and further wherein said step of texturingthe insert sidewall further comprises: etching the insert sidewall toprovide a mottled surface of the insert sidewall.
 15. The method ofclaim 8 wherein said step of texturing the insert sidewall increases thesurface area of the insert sidewall by at least about 30%.